Robert sent in the links – http://crev.info/2017/03/water-theories-evaporate/ … and https://theconversation.com/comets-or-volcanoes-scientists-are-changing-… … which was featured in a post a while back. However, as Peter disagrees with the idea of water with an origin inside the earth it is worth repeating the subject matter – and no doubt many others are baffled by the idea. The old idea was that water arrived from space – comets have been implicated (but as comets have recently failed to live up to their reputation as dirty snowballs we now have an alternative idea). Degassing from within is now the idea and The Conversation does a reasonable job of describing the process. However, it may occur to people that if water can originate from the Mantle (or the lower crust) it is also possibly for water to migrate back to the Mantle. Is this possible? The first link looks at this point in particular.